Do you think this girl liked me? + Why would she stop talking to me all the sudden?

She definitely seemed to like you. But may have felt that it was going no where, so she dated the next guy. He may feel intimidated because of how close you two were, and if he said something to her, she may be trying to detach herself from you so that she doesn't lose her boyfriend. I wouldn't tell her that you liked her and still do now, just because things will get dramatic fast. Wait until she has a falling out with her boyfriend, be there for her, and then tell her. That'll be your best bet.